Likely because you have a very tight chamber in that rifle, and you "neck" or "partial" resize only.
The less the brass case gets "worked'' or full length resized the less stretching, if any is to be seen.
If you're really fussy about accuracy, you would check case neck length every time to make sure they are all within .002 of each other.
Some folks believe this makes a difference in POI from round to round when you're aiming for a tiny dot or center X.
I had one rifle that it did make a difference if the lengths went out of the .002 limit.
Most rifles will easily handle .005 differences and hunting rifles more.
It's sometimes just something else for hand loaders who shoot match rifles to be anal about.